add card icons for valid card types

[notwaldorf]

If the card is valid, its icon is displayed:

I've moved the validating code out of the validator and into the change handler, so that the fancy result (card type/icon etc.) can be used. PTAL

[morethanreal]

lgtm. Another way to do this is to expose the validation response in a bindable property, like iron-ajax.

[notwaldorf]

I've done two small extra changes in validate() :

• it now calls updateAddons(), so that the error message is displayed from manual validation as well (and added a test that failed spectacularly before the change).
• an non-required but empty credit card number is valid.

Is this still ok?